Elongated loads, such as logs, may be transported by means of a truck-trailer unit where the load is contained by two bunks. Each bunk is formed of a generally horizontal bunk beam and vertical bunk stakes mounted at each end of the bunk beam. One bunk is mounted towards the rear of the trailer while another bunk is mounted on the truck.
The bunk mounted on the truck is typically rigidly attached to a hub and saucer or fifth wheel assembly to allow the bunk to rotate with the trailer when the truck-trailer unit goes around curves and corners. However, when a truck-trailer unit carrying a load of logs passes over the crest of a hill or crosses the bottom of a valley, significant stress is placed on the forward bunk if the bunk is rigidly attached to the truck. This stress is a result of the longitudinal axis of the load and trailer being inclined relative to the longitudinal axis of the truck in the vertical plane. In such situations, the saucer may partially disengage from the hub or the front wheels of the truck may actually lift off the road surface, creating significant safety hazards. In addition, the stress causes components of the bunk assembly to wear out prematurely.
The prior art demonstrates previous attempts to solve this difficulty.
U.S. Pat. No. 3,401,955 issued on Sep. 17, 1967 to Alton discloses a logging vehicle with an articulated log bunk. The log bunk is mounted to the logging vehicle by trunnion assemblies which allow pivoting "about an axis extending transversely of the truck . . . whereby the bunk may accommodate displacement of the load from a substantially horizontal position to an inclined position relative to the truck."
U.S. Pat. No. 3,472,527 issued on May 29, 1967 to Keinonen discloses means for mounting a log bunk on a truck. These mounting means includes means which enable the bunk to pivot about a "generally horizontal axis that extends transversely of the truck frame."
However, the mechanisms in Keinonen and Alton have one substantial drawback: the hub-and-saucer assembly, or the means by which the trailer attachment pivots around a vertical axis allowing the truck-trailer unit to negotiate corners and curves, pivots with the bunk around the generally horizontal axis that extends transversely of the truck. As a result, when the truck-trailer unit is negotiating a curve at the same time it is cresting a hill or crossing the bottom of a valley, the load will still create significant stress in the bunk mounting structure. That stress is a result of the fact that the horizontal axis always extends transversely of the truck, rather than extending transversely of the trailer and the load. When the trailer and truck are angled as when negotiating a curve, the horizontal axis about which the trailer attempts to pivot is not parallel with the horizontal axis about which the bunk mounting means allows it to pivot.
Therefore, there is a need in the industry for a bunk mounting mechanism which incorporates a trailer pivot mechanism around a vertical axis as well as allowing the bunk to shift by pivoting around a generally horizontal axis and allowing both pivoting motions to occur at the same time in a relatively stress-free manner.